Of a Kind is growing up and out. The online retailer, which launched a store for limited-edition fashion accessories on top of Tumblr almost two years ago, is expanding its offerings and moving beyond the blogging network that gave it its start.

In late August, Of a Kind launched Collections in private beta, inviting 10 designers to set up their own shops on the site. Around 2,000 people, primarily existing U.S. customers, currently have access. Collections is scheduled to launch publicly next spring.

In an interview with Mashable last week, co-founders Claire Mazur and Erica Cerulo acknowledged that their current model — introducing two to three limited-edition products made exclusively for Of a Kind each week through a series of editorial features — has become too limiting. Often, customers will discover a designer through Of a Kind, and then go on to buy several more pieces from that designer elsewhere. “We’ve been missing out on those sales,” Cerulo explained.

As with other third-party seller marketplaces including Amazon Marketplace, Fab.com and Farfetch.com, sellers are responsible for managing inventory, shipping and returns — Of a Kind, however, is the first point of contact for customer service. Sellers are required to ship orders within 72 hours. So that Of a Kind can track shipments, sellers must use the company’s UPS account. Of a Kind takes an undisclosed cut of each sale that is “far less than wholesale,” Mazur says.

Mazur and Cerulo say Collections isn’t just a good growth model for them, it also delivers an elegant solution for the designers they work with, many of whom come to the pair for advice on setting up and driving traffic to their own e-commerce offerings. “Designers are often asking us how to set up their own e-commerce, or they’re blogging and they don’t know why they aren’t making sales [from it],” Cerulo says. “We’re handling those parts of it — the storefront, the marketing, the editorial.”

I asked Mazur and Cerulo what surprising things they’ve learned about their customer since launching Collections. “We’ve learned that people are willing to spend more [on a single item],” Cerulo said. “We’ve also focused mainly on accessories thus far, and we see that they’re buying a lot more apparel.”

Mazur said they were pleased to see that many shoppers were e-mailing in to ask which items were exclusive and where they were sourced, since the product pages set up by sellers don’t always list those facts. “It really reinforced that those parts of our business are important,” she said.

Tumblr is also still important to the business: Two-thirds of visitors to Ofakind.com have a Tumblr account they log in to least once per week. “Tumblr was where people were getting stuff first — it made sense to target early adopters there,” Mazur says of their decision to launch on the platform.

But Of a Kind has in some ways evolved beyond Tumblr. Product and editorial pages are still posted through Tumblr’s content-management system, but the “heart,” “reblog” and “dashboard” buttons no longer appear on the site.

Mazur and Cerulo say that social media is still important for marketing, but so are other sources, namely their newsletter, which converts the highest. They also advertise on fashion blogs through giveaways and display advertising, which delivers higher ROI than Google AdWords and Facebook ads, they say.

And how close is Of a Kind to profitability? “Certain months we’re profitable,” says Cerulo, but the three-person startup is looking to raise more funding this fall so they can hire more staff.

In the future, Mazur and Cerulo imagine Of a Kind evolving into a “full-service platform” for designers. “We’ll be an incubator plus retail site plus investment firm plus magazine. We’ll be the place where designers come to us to ask about display ads and leather factories. Maybe we’ll even have a showroom.”

Given the connections and knowledge they’ve amassed within the design and tech communities over the past two years, it’s not difficult to believe they’ll do just that.